1. Field of the Invention
The present invention relates to a data receiving device which compensates for transmission loss of a signal transmitted employing a cable or the like, and particularly relates to a data receiving device which equalizes deterioration/attenuation which relies on frequency.
More specifically, the present invention relates to a data receiving device which equalizes deterioration/attenuation which relies on frequency in the event of transmitting high speed digital data over a long distance via a differential signal transmission path, and particularly relates to a data receiving device to perform equalizing processing with high-pass gain compensation employing a reference clock included in the differential signal transmission path.
2. Description of the Related Art
HDMI (High Definition Multimedia Interface) is an interface standard formulated for digital picture and audio input/output primarily for household appliances or AV equipment, and specifically is configured so as to further develop a DVI (Digital Visual Interface) of a digital interface employed for connecting a personal computer and a display and so as to transmit/receive a picture signal, audio signal, and bi-directional control signal together with one cable, simplifying cable connections. Also, the control signal can be transmitted bi-directionally as an option, whereby multiple AV devices can be controlled employing one remote control by relaying between devices.
The HDMI employs TMDS (Transition Minimized Differential Signaling) serving as a digital transmission method for a display picture signal also used for a DVI of a physical layer, enabling realizing high-speed digital data transmission. TMDS is way of carrying out differential transmission for digital data, and is configured with a link made up of a total of four channels, one channel each for the transmission of three types of picture signals called R (Red)/G (Green)/B (Blue), and the reference clock signal. Each picture signal is subjected to serial conversion of a 10-bit parallel signal, whereby 10 bits of data is transmitted for each clock cycle. For example, if the clock is 500 MHz, five gigabits of picture data can be transmitted per second (the actual transmission rate for HDMI ver 1.3 is 250 Mbps to 3.4 Gbps).
TMDS is a digital data transmission method for transporting clock data and NRZ (Non Return to Zero) data serving as a differential signal to a conductor of a pair such as a twisted pair cable. This type of transmission method has advantages such as being resistant to potential differentials of the transmitter, being able to eliminate external noise with a common mode voltage removal operation, and suppression of unnecessary radiation, and can also be employed for high speed data transmission of a comparatively long distance of 10 to 100 meters.
Incidentally, a conductor always has limited attenuation for every unit transmission length, moreover, this appears significantly as to the high frequency components resulting from skin effects or dielectric loss. Therefore, in the event of transmitting high-speed digital data as described above with a differential signal transmission path such as TMDS or LVDS over a long distance, signal attenuation or delay can occur on the transmission path, whereby data waveforms can deteriorate, and the higher the high-frequency bandwidth, the greater the attenuation on the transmission path becomes, which becomes more pronounced the longer the transmission path is.
As a way to resolve such a conflicting relation, providing an equalizer (cable equalizer) to remove the attenuation influence of the high frequency component on the high-speed digital transmission path on the receiving side, and reproducing the signal without conductor attenuation is considered to be extremely important.
As a way to equalize the deterioration/attenuation which relies on frequency (transmission loss: resistance loss and the like from dielectric loss, skin effects and so forth), generally the original transmission signal is reproduced through a high-pass filter subjected to Automatic Gain Control (AGC) so as to have the same amount of gain features as transmission loss. In other words, by applying roughly inverse features as that wherein the high frequency bandwidth is subjected to deterioration/attenuation according to transmission path length with AGC (i.e. apply high-pass gain compensation), obtaining flat features in a wider region and restoring the signal prior to transmitting is desirable.
In such a case, following an arbitrary transmission loss, a receiving signal with the same amount of gain thereto is equalized, thereby necessitating accurately determining the rate of an arbitrary transmission loss. If a transmission loss smaller than the actual cable length is estimated, the feedback voltage becomes smaller so gain compensation is insufficient, whereby the signal having passed through the high-pass filter is subjected to what is known as “under-equalizing”. Conversely, if a transmission loss longer than the actual cable length is estimated, the feedback voltage becomes greater than necessary, whereby gain compensation becomes excessive, and the signal which has passed through the high-pass filter is subjected to what is known as “over-equalizing”.
A system and equalizing method adaptable to automatically adjust an equalizer for compensating the transmission loss relying on the frequency of the transmission signal has been proposed, for example, by quantifying the rate of transmission loss of the sampled voltage with a specified timing at the transition edge between a 1 and 0 of the data signal after attenuation is compared with the data signal common mode voltage (for example, see U.S. Patent No. 2006/0045176).